Working hypothesis There is no difference in long term clinical and microbiological outcome between groups of patients treated with SRP/FDIS and SRP with or without Metronidazole. Treatment outcome is measured by the recurrence rate on an individual as well as on a group level. (see definitions at end-point evaluation - p6), at included sites at 1, 2, 3, 4 and 5 years of follow up. "No difference" is consistent with nil-hypothesis.